The hiss is also dependant on EQ settings. If the song itself has a small amount of background hiss (like old analog recordings transferred later to digital format) it will be present more or less depending on how much the EQ is set on that particular frequency. I noticed this when listening to Vangelis - Heaven and Hell, ripped by me from CD to high quality ogg.
And also, the IEM fit is very important. In the particular case of CX300 (and EP-630) I can't get a proper fit with my ears. Using the small earbuds the highs and mid-highs are too strong and anoying (so the hiss is very audible). On the other hand, the medium earbuds completely seals my ear (it is a little painful to insert them because the air cannot pass outside...) and all I hear is a bloated bass.